by Renata Bailey, Joseph Marchesani, Andrew C. Marinucci, Jennifer Reynard, and Paul Sanders. Abstract. This study examines the effects of sequential ...
Use of Sequential Filtration for Determining Transportable Lead in Ground Water by Renata Bailey, Joseph Marchesani, Andrew C. Marinucci, Jennifer Reynard, and Paul Sanders
Abstract This study examines the effects of sequential filtration on the particle abundance and lead concentrations in ground water from four monitoring wells in New Jersey with a history of high turbidity, elevated metal concentrations, or where differences in metal concentrations exist between filtered and unfiltered samples. In these monitoring wells, both transportable suspended particles, such as colloidal particles that are suspended in solution, and nontransportable particles that are disturbed during sample collection but not considered mobile transportable species may be present in solution with potential overlap in particle size distribution. Filtration, particularly the operational pore size (25 to 0.45 lm) of the filter, was evaluated as a method to obtain a representative sample of the transportable metal, as defined by the dissolved phase and particles that are persistently suspended in solution. Two monitoring wells at the Denzer-Schaefer site, a silty/clay aquifer with high particle concentrations (>8900 mg/L) from samples taken with bailers and a low-flow purge (LFP) pump, showed that a filter of pore size 25 lm could remove 60% to 90% of soil-derived particles, with minimal loss of suspended particles from solution. The two monitoring wells within the highly conductive Picatinny Arsenal sand aquifer provided higher particle abundance with the samples collected with bailers (4300 to 6500 mg/L) than with the LFP pump (4 to 11 mg/L), indicating greater artificial particle disruption with a bailer. At Picatinny Arsenal, the major portion of nontransportable particles in the ground water samples could be removed by filtration through a 25–lm pore size filter, with a minimal loss in suspended particles. Filtration of ground water through a 25–lm pore size filter followed by acidification at the sampling site would provide investigators a tool to examine particle transport in aquifers where there exists the potential for artificial particle disruption during sampling.
Introduction In order to conduct quality research or implement an impartial regulatory program, environmental professionals require ground water samples with reproducible trace metal concentrations that are representative of the concentrations in the aquifer. The monitoring well from which the sample is obtained must provide a representative hydraulic connection to the geologic formation of interest. The lowest trace metal concentration, usually dissolved metal concentration of a ground water sample collected from a particular monitoring well that is determined to be the most representative, is not acceptable in the regulation of ground water as mobile ground water particles (colloids and suspended particles) can contribute to the transport of ground water contaminants (Puls and Barcelona 1989). In order to obtain a representative sample of transportable (mobile) or dissolved metal constituents, an appropriate sample collection method and a filtration protocol must be selected to remove or separate particles artificially
Copyright ª 2005 National Ground Water Association. 52
mobilized during sample collection from a subsample to undergo quantitative analysis. The impact of collecting, analyzing, and evaluating ground water samples with artificially mobilized particles, such as soil-derived particles, would be an ‘‘apparent’’ increase in metal concentration.
Particle Sizes There is a continuum of particle sizes in suspension in a water sample. This includes soil-derived nontransportable particles, persistently suspended particles, and truly dissolved constituents. Soil-derived particles have the largest size, with fine sands ranging from 250 to 10 lm, silts from 50 to 2 lm, and clay 1) for the ground water in the monitoring well (NJDEP 2003; Puls and Barcelona 1996; U.S. EPA 1996, 1998). Seasonal variations in particle abundances and lead solution concentrations are observed at these monitoring wells, and consequently, the bailer samples were collected immediately following the use of the LFP sample to allow for comparison between the sampling approaches. The LFP sample was collected first as it is known to cause less artificial entrainment of soil-derived particles in a monitoring well than a bailer. A 1-L open-tube, bottom-loaded, Teflon, disposable bailer was used for collection of the bailed ground water sample. Table 1 shows a summary of the ground water samples collected at each of the four monitoring wells. To samples
labeled LFP-A, baseline nitric acid (~6 mL/1 L ground water) was added to the polypropylene sample bottles at the field site according to standard procedures (U.S. EPA 1982, 1993), while the other samples (LFP and Bailer) were not acidified prior to filtration. For the bailed sample at monitoring well MW-5, the particle lead levels were determined from the particles collected on filters.
Sequential Filtration and Analysis Vacuum filtration was used as the fractionating technique with sequential filtration of a 1- to 2-L ground water sample through 25-, 10-, 5-, 3-, 2-, 1.2-, 0.8-, and 0.45-lm filters to minimize surface coagulation as shown in Figure 3. Filter media were Whatman 41 for pore size of ~25 lm,
Table 1 Sample Collection at Picatinny Arsenal and Denzer-Schaefer Monitoring Wells Sample Notation
Site Location
OD-2A-LFP OD-2A-LFP-A OD-2A-bailer OD-4A-LFP OD-4A-LFP-A OD-4A-bailer MW-5-LFP MW-5-bailer MW-3-LFP MW-3-bailer
Picatinny Arsenal Picatinny Arsenal Picatinny Arsenal Picatinny Arsenal Picatinny Arsenal Picatinny Arsenal Denzer-Schaefer Denzer-Schaefer Denzer-Schaefer Denzer-Schaefer
Sampling Method LFP LFP Bailer LFP LFP Bailer LFP Bailer LFP Bailer
Acidified Prior to Filtration No Yes No No Yes No No No No No
Analysis SF SF SF SF SF SF SF SF1, PL SF SF
1 No 3-lm filter was used in SF approach. Note: Lead solution and particle abundance analyses notation—SF, sequential filtration through filters of pore sizes 25, 10, 5, 3, 2, 1.2, 0.8, and 0.45 lm; PL, particle lead level.
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Whole Water Recoverable (WWR) Unfiltered 1-2 L Ground Water Sample
0.5-225 mL for determination of WWR
50-225 mL filtrate lead (ppb) Whatman 41 filter paper (> 25m)
>25 µm particles on filter 50-225 mL filtrate lead (ppb)
10.0 µm pore size
>10.0-25 µm particles on filter 50-225 mL filtrate lead (ppb)
5.0 µm pore size >5.0-10.0 µm particles on filter
3.0 µm pore size
50-225 mL filtrate lead (ppb) >3.0-5.0 µm particles on filter 50-225 mL filtrate lead (ppb)
2.0 µm pore size >2.0-3.0 µm particles on filter 50-225 mL filtrate lead (ppb) 1.2 µm pore size
>1.2-2.0 µm particles on filter
50-225 mL filtrate lead (ppb) 0.8 µm pore size >0.8-1.2 µm particles on filter 50-225 mL filtrate lead (ppb) 0.45 µm pore size >0.45-0.8 µm particles on filter
Figure 3. Sequential filtration scheme for ground water samples.
Osmonics Teflon (pure polytetrafluoroethylene) for 0.45, 1.0, 5, and 10 lm, and Millipore Isopure polycarbonate membrane filters for 0.8, 1.2, 2, and 3 lm. Unused filters of each pore size were digested with acids to confirm that background levels of lead on filter media were below detection limits. Samples were stirred prior to each filtration stage to ensure that the entire sample was filtered and stored at 4C prior to analysis. The filter was replaced when clogging of the filter was apparent or when filtration slowed. Typically, 15 to 500 mL volumes were used. In particular, for ground water samples collected with bailers with high turbidity, small volumes (15 to 30 mL) were filtered through early stages (25- or 10-lm filters) prior to replacement with a new filter in order to minimize clogging and sedimentation of particles for the total 1 to 2 L volume. As shown in the sequential filtration scheme in Figure 3, a filter sample containing particles from each stage of the 56
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sequential filtration was used for quantification of particle abundance as defined by the particle mass per volume of solution filtered. Mass of the filter was determined before and after filtration using an MT-5 microbalance in a temperature- and humidity-controlled glove box. Gravimetric analysis of filter samples with pore sizes 25 lm for samples collected with the LFP technique and bailers at monitoring well MW-5 are 8797 and 8935 mg/L and at monitoring well MW-3 are 140 and 602 mg/L, respectively (Figures 4A and 4B). Particle abundances for ground water samples collected at Picatinny Arsenal (Figures 4C and 4D) are significantly different from those for samples collected at DenzerSchaefer (Figures 4A and 4B), having very low abundance of particles >25 lm (4 to 10 mg/L) for samples collected with the LFP at both monitoring wells (OD-2A and OD4A) as compared to higher particle abundance for samples collected with bailers (>4000 mg/L). Highly conductive aquifers, such as that at Picatinny Arsenal, typically provide ground water with less particle abundance. However,
5
5
10
15
20
25
Figure 4. Particle abundance in filtrate of ground water samples. Ground water samples were taken from the following monitoring wells: (A) Denzer-Schaefer MW-5; (B) DenzerSchaefer MW-3; (C) Picatinny Arsenal OD-2A; and (D) Picatinny Arsenal OD-4A. LFP pump and LFP-A preserved with acid prior to filtration.
it has been noticed that even in this highly conductive aquifer, the ground water sample collected with the LFP technique has a small amount of particles present that were probably artificially introduced during sampling. To illustrate the impact of acid addition prior to obtaining a truly representative transportable ground water sample, a set of samples collected with an LFP pump at Picatinny Arsenal were acidified prior to filtration. Under field conditions, a contractor may assume that any sample that was taken with an LFP pump would yield a representative sample and consequently, acidify the sample; however, as illustrated here there is a small percentage of monitoring wells that can yield turbid samples even when the LFP technique is used. Figure 4C shows that acidified LFP ground water prior to filtration (denoted as LFP-A in Figure 4) from Picatinny Arsenal monitoring well OD-2A had lower particle abundance than LFP samples that were not acidified prior to filtration, suggesting a shift in particle R. Bailey et al./ Ground Water Monitoring & Remediation 25, no. 3: 52–62
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size distribution to smaller particle sizes with addition of acid as a result of dissolution of particles. No significant loss of particle mass was observed after acidification of ground water prior to filtration taken with the LFP technique from Picatinny Arsenal monitoring well OD-4A, which was attributed to coarser grain sand particles present at that monitoring well. Particle abundance was also higher at monitoring well OD-4A (~10 mg/L, Figure 4D) as compared to 4 mg/L for particles >25 lm at monitoring well OD-2A. The major fraction of particles in the LFP samples at both monitoring wells OD-2A and OD-4A (acidified and nonacidified prior to filtration) are in the pore size range of >25 to 10 lm. Particle Lead Levels at Denzer-Schaefer Monitoring Well MW-5 The levels of lead in the particles removed during each filtration stage may provide information on the origin of the soil particle. A constant particle lead level is expected for nontransportable particles from lead that is naturally abundant in the soil-derived particles. Transportable particles, although a smaller contribution to total mass, can potentially contain higher levels of lead per mass of suspended particle, but regardless, a difference in particle lead levels between soil-derived and suspended particles should exist. At the Denzer-Schaefer site where clay and silt particles were visible in the ground water taken at monitoring well MW-5, the particles collected after filtration of the ground water were also evaluated to determine the relationship of operational particle size (filter pore size) with particle lead levels to obtain a better understanding of the particle origin. The amount of lead per mass of particles collected on filters is relatively constant for the bailer sample (~85 mg/kg) for particles >2 lm (3 lm was not available for this sample) (Figure 5). The constant level of lead suggests that the major portion of particles in the pore size range of 25 to 2 lm is of the same origin, namely, silt/clay–derived sediment particles that were observed to have a grayish appearance. Results also show that the amount of lead attributed to particle-associated lead of each operational pore size in the filtrates is greatest with the 25-, 10-, and 2-lm filters (when no 3–lm pore size filter used) as these filters remove the greatest mass of silt/ clay–derived particles (Figure 5). Filtration through 1.0– or 1.2–lm pore size filters removes particles that are yellowbrown in appearance, similar to those observed on the 2-lm filter for the LFP sample. These particles have higher lead levels per mass of particles (Figure 5). Although by definition clay particles can exist below 2 lm, the large change in lead per mass of particles indicates the presence of suspended particles rather than clay particles. The amount of lead associated with suspended particles below ~2 lm is relatively small in comparison to silt/clay–derived particles as the total mass of particles 700 ppb at both monitoring wells because of the presence of increased amounts of large particles, >25 lm, disturbed using a bailer as compared to the LFP technique. After filtration of the bailed ground water sample from monitoring well OD-2A through a filter of 25-lm pore size, the lead solution concentrations decrease to 90% of particles associated with soilderived sediment that are artificially disturbed during sample collection. In some cases, the resultant filtrate was found to be compliant with current regulations for the State of New Jersey and the Federal Government (10 ppb). The implementation of sequential filtering can be used in an initial investigation of an aquifer as a tool to determine whether a heavy metal contaminant, such as lead, is associated with particles of a potential colloidal nature (10 lm). If the latter case is observed, then filtration of samples through a single pore size membrane (8, 10, or 25 lm or larger) would be justified and would reduce the sampling time in comparison to LFP techniques. Furthermore, such results could eliminate the need for long-term monitoring if the filtrate concentrations were found to comply with applicable ground water standards. In addition to use with bailer-collected samples, the filtration analysis described in this paper would be applicable to samples collected from temporary drive-point locations. If filtration of a bailed sample can be reliably used in place of LFP, the time of well sampling could be reduced from 24 to
